1. Field of the Invention
The present invention relates to an optical network system, optical network device and optical network control method to execute data communication through an optical network.
2. Description of the Related Art
Recently, data communication through an optical network has been popularized with the purpose of remote control of electrical equipment. To execute data communication through an optical network between one piece of equipment as a master unit and multiple pieces of equipment as slave units, as published in Unexamined Japanese Patent Application KOKAI Publication No. H8-147229 for example, this equipment is interconnected in the shape of a so called multi-drop connection to execute data communication. The disclosure of the Unexamined Japanese Patent Application KOKAI publication No. H8-147229 is incorporated herein by reference in its entirety.
For this multi-drop connection, an order is previously given to the multiple pieces of equipment as slave units, and the master unit and the slave unit at the highest rank are first connected through an optical transmission line such as optical fiber. Next, the slave unit at the highest rank and that at the second highest rank are connected through an optical transmission line. Thereafter, down to the slave unit at the lowest rank, adjacent slave units are mutually connected through the optical transmission line, resulting in the formation of an optical network in the shape of a chain.
In an optical network in the form of a multi-drop connection, data communication is generally executed by a method called the polling system, whereby the master requests each slave to respond, and the slave returns the response. The data communication between the master and the slave is generally executed by synchronizing to a clock signal generated by the master and slave independently.
Actually, when transmitting data from the master to the slave, for example, the master transmits the data to the slave by synchronizing with the clock signal generated by the master itself. The slave latches the data transmitted by the master in synchronization with the clock signal generated by the relative slave itself for receiving the data. When the slave transmits data to the master, the slave transmits the data to the master in synchronization with the clock signal generated by the slave itself. The master latches the data transmitted by the slave in synchronization with the clock signal generated by the master itself for receiving the data.
Similarly, data communication between multiple slaves may be mutually executed. In practice, when the data received by the first slave from the master is addressed to the second slave, for example, the first slave transmits the data to the second slave in synchronization with the clock signal generated by the first slave itself. Further, the second slave latches the data transmitted by the first slave in synchronization with the clock signal generated by the second slave itself to receive the data.
However, when the transmission of a pulse representing data is executed in synchronization with the clock, there will be a problem with the deviation of time length or phase of the pulse, as the pulse is latched under an unstable state of pulse transition or the like. Assuming that the time length of the pulse constituting the clock signal generated by the master is “1”, when a pulse with a time length of “5” is transmitted from the master to the slave, a situation could arise, in which for example, the time length of this pulse transmitted at the time of receiving by the slave may become a time of “4” or “6”, or the phase may deviate for one clock cycle.
As a method to solve such a problem, it has been considered to reduce the fluctuation of the time length of pulses representing data, or that of percentage phases by making the time length of pulses representing data sufficiently longer than the time length of pulses constituting the clock signal.
Another method includes the slave receiving a pulse storing the data sampled in the cycle of clock signal into the stack of FIFO (first-in, first-out) type, and reproducing the pulse by reading the data from the stack. When the time length of a pulse unexpectedly takes a longer or shorter time length, (although the time length of a pulse should be an integer multiple of five in a clock cycle, when another value is employed, for example), deviating the reading position from the stack may reproduce a pulse in which the time length was corrected (the pulse of an integer multiple of five in a clock cycle having the time length closest to that before correction, for example). When the reproduced pulse is supposed be transmitted to the yet succeeding slave, the slave reproducing the pulse transmits the pulse to the relative succeeding slave.
Despite of employing the above mentioned method using a stack, however, a problem with the deviation of the time length or phase of a pulse has been generated due to the jitter of the clock signal or fluctuation of frequency. In addition, making the time length of a pulse representing data as long as possible to withstand practical use has not been successful in removing the effect of the deviation of the time length or phase of a pulse.